I have a large inut raster geotiff map of size 12000x9000. I am running r.watershed to output a stream raster map. However it is much lower resolution, only about 960x711. How do I make GRASS GIS / r.watershed command output a matching size raster map of same resolution of my input?
In general, GRASS GIS is ready for massive data analysis.
In your case, you need to use g.region, by using the corresponding entry in the map legend context menu (right mouse button on legend entry) or on command line (generic example):
g.region raster=your_raster_map -p
It will then use bounding box and raster resolution to set the computational region.
For the underlying concept, see https://grasswiki.osgeo.org/wiki/Computational_region .